Filter for electric line system



April 2, '1957 H. SCHEFTELOWITZ FILTER FCR ELECTRIC LINE SYSTEM Filed June 22, 1954 2 Sheets-Sheet l ig.2C

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m 4 rim/W9 m 7 W3 7 M 6 H April 2, 1957 H. SCHEFTELOWITZ 2,787,766

FILTER FOR ELECTRIC LINE SYSTEM 2 Sheets$heet 2 Filed June 22, 1954 Fig. 8

FILTER FOR ELECTRIC LINE SYSTEM Henry Scheftelowitz, Stockholm, Sweden, assignor to Telefonaktieholaget L M Ericsson, Stockholm, Sweden, a corporation of Sweden Application June 22, 1954, Serial No. 438,549

Claims priority, application Sweden June 30, 1953 4 Claims. (Cl. 333-73) This invention refers to a filter for electric line systems.

A filter according to the invention comprises a line system where b: is the value (with its sign) of the middle suscep tance, b1 is the value (with its sign) of each of the outer susceptances and a is defined as Af f were I is the middle frequency of the filter, M is half the band width between the tops of the attenuation curve of the filter (i. e. the curve showing the attenuation of the filter as a function of the'frequency) and k is a number, approximately 3. In diflferent cases k may vary from 3.0 to 3.3.

The invention will be more closely described with reference to the accompanying drawing, where Figs. 1a and b show a sectional view of a wave guide with a filter according to the invention, and Fig. 1c shows the corresponding equivalent diagram, incoming susceptances being negative; Figs. 2a, b and show another wave guide with a filter according to the invention and with the corresponding equivalent diagram, incoming susceptances being positive; Figs. 3 and 4 shows a sectional view of a coaxial conductor with filters, Fig. 5 shows a two-wire line with shortcircuit lines, and Figs. 6-9 show some diagrams as a basis for a theoretical'origin.

In the wave guide according to Figs. 1a and b there are three thin plates inserted one after the other across its longitudinal direction, in which plates a window is arranged, the longitudinal direction of which coincides with the direction of the short sides of the wave guide. These plates work as negative susceptances, and the dimensions of the windows are so chosen, that the value be of the middle susceptance is dependent on the 'value b1 of each of the outer, identical susceptances according to the relation:

where a is equal to The equivalent diagram according ul-Fig. '10 for the wave guide according to Figs. la and b shows three inductances connected in parallel-in a'line system which is terminated by its characteristic impedance, in this case thought of as solely resistive.

In the wave guide according to Figs. 2a and b three thin plates are inserted one after the other across its longitudinal direction in which plates a window'is arranged, the longitudinal direction of which coincides with the direction of the long sides of the wave guide. These plates work as positive susceptances, and the dimensions of the windows are so chosen, that the value b2 of the middle susceptance is dependent on the value hr of each of the outer, identical susceptances according to the relation (1) above with The equivalentdiagram. according to Fig. 20 for the wave guide according to Figs. 2a and b shows three capacitances connected in parallel in a line system, which is terminated by its characteristic impedance, also in this case thought of as solely resistive.

In the coaxial conductor according to Fig. 3 the inner conductor 30 is connected tothe outer conductor 33 via three conductors 31, 32 and 31, arranged across the inner conductor and through holes in the outer conductor, said three conductors being at their outer ends short-circuited to tubular branches of the outer conductor 33. The length m1 and m respectively, of these branchings decides the kind and the size of the susceptances, forming the three conductors, and they are to be chosen so, that the Relation 1 will be valid.

In Fig. 4 a coaxial-conductor is shown, in which three susceptances have been formed by annular recesses in three plates 41, 42 and 41, respectively, perpendicularly arranged to the longitudinal direction of the coaxial line. The two outer plates are equal, the recess of the middle plate being smaller than the one of the outer plates. The annular recesses are to be dimensioned so, that the Relation 1 will be valid for the three susceptances with a In the two-wire line according to Fig. 5 three shortcircuit lines with the lengths 11, 12 and 11, respectively, are connected in parallel between the branches of the line. The lengths 11 and 12 are so chosen, that the Relation l is valid for the three susceptances which are formed by the three short-circuit lines. The line is terminated by its characteristic impedance.

For arrangements according to Figs. 3 and 5 it is valid that a is equal to when the susceptances are negative, and equal to when the susceptances are positive.

In the described filters the outer susceptances (jbi) are situated at a distance of about 1 from the middle susceptance (ibz) where 1 is given by the relation in which relation A represents the wave-length, the corresponding frequency of which lies in the middle of the pass band of the mentioned filters. In practice it has 'proved the scope of the invention being departed from. Thus,

suitable, in some cases, to make the resonators of the cavity about 1% shorter than ,as according to the mentioned relation, and to efiect tuning by means of a screw.

Many other embodiments may be thought of without to be dimensioned according to the same rule as mentioned above. T he above mentioned relation between the susceptan'ces in a filter according to the invention canbe' derived theoretically according to the following:

An equivalent admittance'diagrarn of a.filter according to the invention is shown in Fig. 6, where three susceptances, jbr, jbz and jb are connected in parallel between the 'two'branches of a line system at the distance 1 from each other, the line system being terminated by its characteristic admittance yo. The diagram according to Fig. 6 may be altered into the equivalent diagram according to.Fig. 7 by adivision of the susceptance jbz into two susceptances output is terminated-by the same admittance.

In Fig. 9 the magnitude P mnx is shown as a function of the frequency f of the system I according to Fig. 8 with'the coupling .factor between the oscillating'circuits as a parameter'Pmsx representing the maximally withdrawable etfe'ct and P the derived eifect. Curve 1 is valid for less than critically coupled circuits, curve 2 for critically coupled circuits and curve "3 for more than critically coupled circuits.

It can be proved that the system according to Fig. 8 can be reduced to a system according to Fig. 7 as regards the frequency characteristic.

In a section to the rightyin'FigJ7 there isa certain admittance, the value of which may be written .A+ 'B. If, to said admittance, is added an admittance with the value and if Mi nis in morethan critically connected circuits equal. to -:A,.

i. c. solely resistive for two frequenciesjr .and :fz,-:one condition for the calculation of b2 is thereby. obtained.

In a section z-z of Fig. 7. the relative admittance is l+jb1, and directly to the right of the section yy the relative admittance is, according to a wellknown linetheoretical formula:

:4 If the frequencies f1 and f2 are given, the following relation for-"determining b: 'isthus obtained 2 b -cos ZQ- -sin 26 b J 'i'J =J' ;"i-J- =0 1 cos"20*b sin 20 which through 'trigonom'etrical transcriptionscan be reformulated to:

b3111 'll/ (50S 20+1l =w (6) 2) 1/( +em an enn a) where 32 ten liq-- "F1 2 tan 0;- 1

Atwmor'e than critical couplings it -is-valid *thatthe Equation' will have-two solutions i. e.

1 4% min-e,

da)( iin) which gives bi i= {1-9; (8) Equation 6 gives 1 1 tan 1/ 3IOC0S-f: '--*=i 29 IP 9) "VG-P19831 t 2)( a i 1) PHI/2) "For practical'purposes'it is suitableto develop in series theEqua'tion '6. At absolute band widths of less than about -Mcrwithinthe C-ba'n'd it is necessary, for a maximum. fault of 5% to take only the terms:

. '2 2 "tan and tan in: b2

wherefroin is obtained:

-2 -2 1/ a a. r 2 1 1 2a+arctan b2 V 13b! (10) t' z -As already mentioned abovetwo solutions corresponding-to"the"'frequencies'f1 and f2 and the wave-lengths M1 and A respectively,.are obtained at morethan critical coupling,"said wave lengths being determinted by "the expression: u g g 1 2 2 1 2 z 2 ,z[-.-. r 11 T All. AR- b2 i 4 b1 wherelLis. thet lengthof .aresonator, see Figno. At absolute bandwwidthstofi the :size- 20400 Mcwwithin the C-band,sit'.-is possible-witham'aximum fault of 4%, to write 21L A, by means of which. 11 changes to f T b. g 4 b; (12) where Af is the distanceofithe frequency between the tops of the frequency function according to Fig. 9.

. .If therdesignation' a is insertedfor f and seine1.n'ansfortuations-.aremade,v 12:.chmges1o {fi s-mm mama-3 I claim:

1. A symmetric line filter for more than critical coupling, comprising in a line system three susceptances connected one after the other characterized by the value of the middle susceptance depending on the value of each of the outer susceptances according to the relation where 122 is the value of the middle susceptance, In is the value of each one of the outer susceptances and a is f being the middle frequency of the filter, A half the band Width between the tops of the attenuation curve of the filter and k approximately equal to 3.

2. A symmetric line filter according to claim 1, comprising in a line system three susceptances connected one 6 after the other, characterized by the incoming susceptances being negative and k being equal to 3.0.

3. A symmetric line filer according to claim 1, comprising in a line system three susceptances connected one after the other, characterized by the incoming susceptances being positive and k being equal to 3.3.

4. A symmetric line filter according to claim 1 char acterized by the outer susceptances being situated approximately at the distance 1 from the middle susceptance,

10 where l is obtained from the relation in which relation A represents the wavelength, the corre- 15 spending frequency of which lies in the middle of the pass band of the mentioned filter.

References Cited in the file of this patent UNITED STATES PATENTS 2,585,563 Lewis et al. Feb. 12, 1952 

